(Best) Dual CPU configuration for PowerEdge T430

I have 2 Xeon E5-2696 v3 that not seems to work properly in dual mode (whereas they are working in standalone mode).

Can you confimr? And why?

I have 2 Xeon E5-2620 v3 that works fine in dual CPU mode...

Does the Xeon E5-2699 v3 works fine in dual mode on my T430?

What would be the best dual CPU configuration for Dell PowerEge T430?

Other question: where can I found the list of Graphic cards that are compatible with my server?

Thanks in advance for your help